1. Field of the Invention
A method to control the pretilt angle of a liquid crystal device (LCD) is disclosed, more particular, the method utilizes a particle beam with plasma or ions to treat the surface of the vertical alignment layers of the LCD before the alignment process, so as to adjust the pretilt angle of the liquid crystal (LC) molecules.
2. Description of Related Art
Liquid crystal is often used for display screens since it has intrinsic photoelectric characteristics. Generally, productive flat-panel LCDs are TN (Twisted Nematic) types.
The basic structure of a TN LCD includes upper and lower conductive glass substrates, with a Nematic liquid crystal injected therebetween. Moreover, two polarized plates with orthogonal-polarization are disposed on the outside of each substrate. Next, an alignment film is coated on each conductive substrate and rubbed with rubbing clothes, so that the liquid crystal molecules are aligned in the rubbing direction. Under conditions in which an electric field isn't being used, the polarized direction of the ray will change as torsion occurs to the liquid crystal molecules after entering the LCD, thereby causing a bright state. Conversely, the liquid crystal molecules will be homeotropic, namely vertical to the direction of the alignment film, when an electric field is used, so as to form a dark state since the ray cannot pass through the second polarizer.
LCD is well-developed, but there are still several problems existing such as the viewing angle, contrast, and display uniformity. Particularly, the above-mentioned alignment film is relevant to implementing high-density and large-sized LCDs, as well as faster response times. Liquid crystal molecules of the LCD are used in displayers since the direction of the polarization of ray can be changed by an external electric field. Shutting off the electric field recovers the original arrangement via the recovering force, i.e. the anchoring force. Therefore, whether the ray passes through the device determines either a bright state or a dark state of the LCD.
There are three kinds of liquid crystal arrangements—homogeneous alignment, vertical alignment, and tilted-angle alignment. Reference is made to FIG. 1A showing the homogeneous alignment. The long-axes of the liquid crystal molecules 14 are parallel with the alignment film 12. The vertical alignment shown in FIG. 1B shows the long-axes of the liquid crystal molecules 16 being vertical to the alignment film 12. The tilted-angle alignment shown in FIG. 1C shows a schematic diagram of a pretilt angle formed between the liquid crystal molecules 18 and the surface of alignment film 12. The structure of pretilt angle is implemented to accomplish a uniform alignment. The pretilt angle is an important parameter affecting the LCD characteristics. Particularly, besides the pretilt angle being formed by the interactive force caused from the chemical structure between the liquid crystal and the alignment film, the morphology of the alignment film also affects the pretilt angle.
Conventional liquid crystal alignment method is rubbing. Reference is made to FIG. 2 showing a surface treatment method on alignment film. A substrate structure 2 is placed on a movable table 20, wherein a glass substrate 4 and an alignment film 6 are included, and rubbing cloth 22 covers the surface of a rolling wheel 24. As the table 20 moves the rolling wheel 24 drives the rubbing cloth 22 to induce surface anisotropy of the alignment film 6.
In the field of non-contact alignment, such as photo-alignment, a polarized UV ray is used to perform surface anisotropy on alignment film. Reference is made to U.S. Pat. No. 5,711,999 illustrating a method of applying light to produce surface anisotropy of a photosensitive alignment film. U.S. Pat. No. 5,711,999 has disclosed the range of light wavelength and the quantity of radiation, which react with the chemical components of the film. The characteristics of the alignment film are changed by controlling the intensity of exposure and the wavelength of the light. The pretilt angles of the LC molecules are changed thereby. Moreover, U.S. Pat. No. 5,623,354 has disclosed that an ultraviolet light can change the pretilt angle of the liquid crystal molecules on the alignment layer of the LCD. A display device with a multi-domain liquid crystal layer is thereby manufactured.
U.S. Pat. No. 6,852,374 has disclosed a LCD, an optical element, a method of fabricating the LCD, and a method of making an optical element. This patent relates to the structural design and a manufacturing method of a liquid crystal panel with optical compensation birefringence (OCB) mode.
For implementing the OCB structure mentioned above, the control of the pretilt angle (i.e. generally, larger than 5 degree but less than 85 degree) of the LC molecules is necessary. For controlling the pretilt angle, the above-mentioned patent uses the wavelength and the quantity of the light source to treat the surface on the alignment film. Since a particular material is used for the alignment film, only specific side-chain of the alignment film will be broken under a specific light wavelength, so as to change the pretilt angle of liquid crystal.
Reference is made to FIG. 3 of the conventional art. An alignment film material 31 includes a main chain structure 32 and a side chain structure 34. A photosensitive structure 33 is used to connect the main chain and the side chain. In general, a light of a specific wavelength corresponds with a specific photosensitive structure 33. When the light of a specific wavelength illuminates the photosensitive structure 33 featuring a specific wavelength, the photosensitive structure 33 is broken, thereby the pretilt angle of the LC molecules is controlled.
In a displaying mode with the optical compensation birefringence (OCB), a high pretilt angle of the liquid crystal molecules is required so that a step of converting a splay state into a bend state of the liquid crystal molecules by stressing an external high voltage can be ignored. However, in view of other conventional arts, if the step of alignment is solely achieved by rubbing, an ionic beam, light beams or plasma beams individually, the high pretilt angle required by OCB will not be reached.
Alternatively, although combining the vertical and horizontal polyimides of the conventional art allows the high pretilt angle to be reached, it requires particular solution mixture and the evaporation process to control the surface fraction of vertical and horizontal polyimides. Moreover, the step of treating the surface by a light or a heat source with assistance of the step of rubbing to produce high pretilt angle will cause a high residual DC voltage or a bad thermal stability.
Consequentially, the technology of the conventional art using light to treat the surface cannot generate the required characteristics of the alignment film precisely because the quantity of light is not easily controlled, so the surface of the alignment film is damaged after exposure. Specially designed alignment film cannot achieve a stable high pretilt angle and is not cost effective as well.